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Testing the Mind's Eye

It's a decades-old question in cognitive psychology: Does the brain process an imagined object the same way it does a real one? A report in tomorrow's Science may provide the long-sought answer. The findings suggest that the primary visual cortex, the first part of the cerebral cortex to receive retinal information, lights up during our daydreaming as well.

Stephen Kosslyn and his colleagues at Harvard Medical School began exploring the brain's strategies for imagining--as opposed to viewing--a scene more than 20 years ago. In his early experiments, Kosslyn measured the time it took people to shift their attention from one feature in an imagined panorama to another. That time lengthened with the distance between features, suggesting--but not proving--that the brain pans across an imagined scene depicted with the same spatial topography as a retinal image. Recent imaging experiments have provided further support that the primary visual cortex (V1) encodes images this way.

Kosslyn's team had already shown that V1 is activated during visual imagery, but to establish that its activity is necessary for the imagery, Kosslyn and Alvaro Pascual-Leone of Boston's Beth Israel Deaconess Medical Center and their co-workers turned to transcranial magnetic stimulation (TMS), which focuses a magnetic field on targeted brain areas, inducing electrical currents that disrupt their functions temporarily. The team applied low-frequency TMS to V1 in eight subjects. Afterward they had the volunteers compare the lengths of pictured bars, either while looking at the picture or while holding its image in memory. TMS, the researchers found, impaired the subjects' abilities at both perception and imagery when compared to a sham treatment.

"This is a very exciting finding," says cognitive neuroscientist Randy Buckner of Washington University in St. Louis. If TMS works as it seems to, he says, it is "exactly what the field needs, an ability to safely manipulate cognitive processing in humans." But he and others warn that the results should be considered preliminary, as the effects of low-frequency TMS are not well understood, and the treatment may affect other brain areas near V1, or even cause a fleeting general disruption of brain function.